thailand, officially the Kingdom of Thailand, formerly known as Siam, The country is a constitutional monarchy, headed by King Rama IX, the ninth king of the House of Chakri, who, having reigned since 1946, is the world's longest-serving current head of state and the longest-reigning monarch in Thai history.The king of Thailand is titled Head of State, Head of the Armed Forces, Adherent of Buddhism, and Upholder of religions.